Export symbols needed by the Chromium component build (part 8).

This CL uses RTC_EXPORT (defined in rtc_base/system/rtc_export.h)
to mark WebRTC symbols as visible from a shared library, this doesn't
mean these symbols are part of the public API (please continue to refer
to [1] for info about what is considered public WebRTC API).

[1] - https://webrtc.googlesource.com/src/+/HEAD/native-api.md

Bug: webrtc:9419
Change-Id: Ib2c29054b2ae008f5291bd3b762a504b18534326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/130513
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27410}

media/engine/simulcast_encoder_adapter.h

@@ -23,6 +23,7 @@
 #include "modules/video_coding/include/video_codec_interface.h"
 #include "rtc_base/atomic_ops.h"
 #include "rtc_base/sequenced_task_checker.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -33,7 +34,7 @@ class VideoEncoderFactory;
 // webrtc::VideoEncoder instances with the given VideoEncoderFactory.
 // The object is created and destroyed on the worker thread, but all public
 // interfaces should be called from the encoder task queue.
-class SimulcastEncoderAdapter : public VideoEncoder {
+class RTC_EXPORT SimulcastEncoderAdapter : public VideoEncoder {
  public:
   explicit SimulcastEncoderAdapter(VideoEncoderFactory* factory,
                                    const SdpVideoFormat& format);